Centrifugal fans are utilized in a wide variety of applications where efficient movement of air is required. In the air conditioning industry, for instance, centrifugal fans provide the energy to move air that has been either cooled or heated by a heating, ventilation, and air conditioning (HVAC) system. The air is transported to the space to be heated or cooled through ducts or other apparatus that form the air delivery side of the HVAC system. In certain applications, the fans are integral with the various components of the HVAC system, as for example, in installations including the fan as part of a single fan coil unit along with coils, filters, air exchangers, and the like.
Centrifugal fans utilized in HVAC systems typically have a circular impeller having structure forming an inside diameter and an outside diameter with a plurality of radially directed blades disposed therebetween. The blades are curved forward in the direction of rotation at the heel of the blade and may be straight or curved slightly away from the direction of rotation at the tip of the blade. An air inlet chamber is defined within the inside diameter of the impeller. The impeller is typically mounted on a stub shaft which is in turn connected to an electric motor. The stub shaft may be an extension of the motor shaft or the motor may drive the impeller through a gear set or by means of a belt and pulleys.
The impeller is typically carried within a scroll-shaped housing. A shroud portion of the housing surrounds the impeller and expands in the radial dimension from a very small to a large cross sectional area, thereby defining the scroll shape. This expansion commences at the base or inner side of the exit port. As the shroud expands around the impeller in scroll fashion, the shroud ultimately forms the air exit port. The expanding scroll-shaped housing contributes substantially to the depth dimension of the fan and limits the applications in which such a fan may be employed. The air exit port has a relatively large cross-sectional area and is typically formed tangential to the circular impeller.
One side of the typical fan housing has a large central inlet opening that is usually circular. The central opening defines an air inlet into the center, interior air chamber of the impeller. In operation, the impeller draws air through the air inlet into the air chamber. The air is drawn into the inlet of the blades. The blades attack the air and accelerate the air in the radial direction. The accelerated air is discharged at relatively high velocity radially from the discharge portion of the impeller blades into the scroll-shaped housing that surrounds the impeller. The high velocity air travels through the air passageway defined by the scroll-shaped housing and is then discharged through the exit port to either heat or cool a space.
For optimum efficiency of the fan, the air passageway defined scroll shroud portion of the fan housing should have a 10.degree. to 15.degree. expansion commencing proximate the base of the exhaust port and expanding as the shroud wraps around the centrifugal fan until the scroll ultimately forms the exit port. This is required in order to efficiently accommodate the ever increasing volume of accelerated air that is forced into the air passageway by the fan. An additional parameter to be considered when designing for optimal performance of a centrifugal fan is the axial fan dimension, which defines the overall width of the fan. In particular, the width should be approximately 120% of the fan diameter. It is known that airflow capacity increases with fan width up to a width that is approximately 120% of fan diameter. With essentially a free inlet condition in which there are no impediments to the free flow of air into the air inlet, the range of design parameters indicated above provides optimal air output of a given centrifugal fan.
It has been noted, however, that the performance of a fan is degraded when less than free inlet conditions are encountered by a system having the above design parameters. A point of diminishing returns is reached where increases in fan width will not provide an increase in airflow, in fact, the airflow may actually decrease for widths too great. A specific application of the fan that results in a restricted airflow to the fan air inlet affects the maximum effective fan width.
More particularly, and as indicated above, centrifugal fans are often utilized to provide the air output from fan coil units. Fan coil units are typically relatively small units that can be utilized to either heat or cool a space. Accordingly, a basic fan coil unit contains a coil in which either hot or cold fluid is pumped, an air filter, and one or more centrifugal fans. A design goal of such units is to keep them relatively small. The small size of a fan coil unit makes such units attractive for remodeling or where existing building structure makes it difficult to install other types of units. The fan coil units are typically mounted on the floor, on a wall, or suspended from the ceiling of the space that is to be heated or cooled. Of particular interest in the design of such units is minimization of the depth dimension. The depth dimension defines the distance that the fan coil unit projects from the wall, ceiling, or other surface that the unit is mounted on. By minimizing the depth dimension, the fan coil unit may be installed in a relatively narrow opening and occupy the minimum possible volume in the space. Additionally, fan coil units typically have a relatively wide, slender air exhaust. In order to provide even air flow across such an air exhaust, many have between one and five fans arrayed side by side across the width of the air exhaust. Such fans may be driven on a common central shaft or two such fans may be mounted on either side of a drive motor having drive shafts projecting on either side thereof. The necessary plurality of fans adds to the complexity and cost of fan coil units.
Minimizing the depth of the fan coil unit has a direct impact on the centrifugal fan that is utilized within the unit. The centrifugal fan utilized within a fan coil unit is typically oriented such that the axial dimension of the centrifugal fan is parallel to the length dimension of the fan coil unit. This orientation means that the diameter of the fan and the fan housing is constrained by the depth dimension of the fan coil unit.
Additionally, the air inlet efficiency to the centrifugal fan is substantially affected by the depth dimension of the fan coil unit. The structure of the fan coil unit that includes the depth dimension effectively forms a duct through which any air entering the air inlet of the centrifugal fan must pass. The reduced size of the depth dimension forms an air flow restriction when compared to the optimal free inlet condition for which such fans should be designed. In the past, such air flow restrictions and dimension restrictions have required the redesign of the centrifugal fan housing to provide the best possible efficiencies. Such redesigns were a compromise, balancing air output with the limited depth dimension of the fan coil unit. Typically the redesigns resulted in only a 3.degree. expansion of the scroll shroud and limited the axial dimension of the centrifugal fan to less than the diameter of the fan. Such minimal expansion does not provide the increasing area of the air passageway that is necessary to accommodate the full volume of air that is capable of being produced by an optimumly designed fan. The redesign reduces the air output of each individual fan, frequently requiring that additional fans be provided in a fan coil unit.
U.S. Pat. No. 3,796,511 discloses a centrifugal fan with a scroll housing that incorporates a dust-skimming slot at the outer periphery of the scroll housing approximate the exhaust port. To insure that air entering the air inlet did not pass directly through the fan and out the dust-skimming slot, a baffle was disposed on the interior side of the centrifugal fan in the vicinity of the dust-skimming slot to prevent such undesired air flow.
U.S. Pat. No. 4,573,869 discloses a centrifugal fan mounted within a relatively large plenum chamber. A first wind direction plate is mounted on the outer surface of the housing, and extends substantially to the center of the air inlet, to straighten the flow of air into the air inlet of the centrifugal fan housing. A second wind direction plate extends from the first plate through the air inlet into the fan along the axial and radial directions of the fan.
U.S. Pat. No. 4,680,006 discloses a centrifugal fan having a standardized production scroll housing. The design includes a radial wall barrier that extends into the air inlet to stabilize the outlet air flow from the fan. While each of the above mentioned patents deal with alterations of centrifugal fan housings to effect changes in fan air flow, none of the patents deal with the problem of optimizing air flow while reducing the depth dimension of a fan such that the fan may be utilized in a confined enclosure such as a fan coil unit.
A centrifugal fan that maximized air flow, while at the same time had the reduced fan depth necessary to accommodate a reduced depth dimension of a fan coil unit would provide decided advantages to the industry. Such an improved fan should include an impeller that is in the optimal range of 120% of the impeller diameter. A fan of such relatively greater width would permit reducing the total number of fans necessary to provide the airflow across the full width of the fan coil unit air exhaust and the expansion of the fan's scroll should remain between approximately 6 and 10 degrees, all within the confines of a compact design.